Compression springs having a helical or coiled shape are well known in the art, and have been used in suspension systems for motor vehicles and the like. The wire used to create the coil spring can be of constant, non-tapered diameter, or can be of tapered form having a smaller diameter at one end of the coil spring and a larger diameter at an intermediate portion. In many applications, such as motor vehicle suspension systems, it is desirable to place a tubular sleeve or hose over a portion of the coil spring having a tapered wire diameter. Such a hose or sleeve can reduce noise as the coils of the spring tend to bump against one another, and can, if properly applied, also protect that portion of the spring against corrosion.
U.S. Pat. No. 3,161,407 (Robin) discloses a coil spring having a sheath of resiliently deformable material covering the convolutions of the coil spring. This sheath can be formed with splines, corrugations, or other relief patterns on at least one of its faces, which will tend to function as a shock absorber as the coil spring deflects. U.S. Pat. No. 3,711,917 (Baumgras) discloses a coil spring which is encased in a coating of protective material, such as Teflon.TM.. Baumgras discloses the use of a Teflon.TM. hose or tube which has a larger inner diameter than the coil spring wire's diameter, so as to leave an air gap between the Teflon.TM. and the spring wire as the Teflon.TM. is slipped onto the wire. After that has occurred, the entire assembly is heat-treated to shrink the Teflon.TM., thereby fitting tightly around the wire.
U.S. Pat. No. 4,763,882 (Nischiyama et al.) discloses a coil spring which has its wire coils coated with a soft resin foam, such as polyurethane foam. This coating is formed by placing the coil in a mold, then forcing expanding foam material into the mold. The disclosed purpose of this design is to prevent an increase in the dynamic spring constant, and to suppress surging of the spring.
U.S. Pat. No. 2,766,806 (Rothermel et al.) discloses a method for making corrugated flexible hoses. Rothermel '806 discloses the use of an outer layer of plastic which is reinforced by an inner coil spring. A differential air pressure is used to urge the plastic (while in a plastic flow state) down onto the mandrel used during assembly, thereby forming corrugations. Another Rothermel patent, U.S. Pat. No. 2,782,803, discloses the use of a flexible conduit having a reinforced interior, in which the reinforcement material is a helical spring which is coated with plastic. Rothermel '803 discloses an assembly procedure such that, after the helical spring is installed inside the tubular sheath, the entire assembly is heated in a chamber. Again, air pressure or a vacuum is used to create the corrugations. In both Rothermel patents, the wires of the coil spring appear to be non-tapered (i.e., having a constant diameter).
Rothermel '806 also discloses the use of a vacuum forming tube to assist in the assembly of the outer cover tube over the outer cylindrical diameter of the coil spring. Alternatively, Rothermel '806 discloses the use of "blowing" the cover tube over the cylindrical coil spring as the coil spring sits on a mandrel.
U.S. Pat. No. 4,869,471 (Schwarz et al.) discloses the use of a tapered hose for covering a portion of the tapered wire of a progressive characteristic helical compression spring. This tapered hose is manufactured by using a drawing mechanism having precise speed control so that the hose can emerge from an extruder at varying speed, thereby creating the tapered diameter. As an alternative, Schwarz et al. disclose a method for manufacturing the tapered hose by keeping the speed of the drawing mechanism at a constant, and varying the rate of extrusion. In both cases, the hose is formed having a varying diameter, in which the diameter at one end is smaller than at the opposite end (thereby forming a "tapered" hose).
Schwarz et al. disclose a preferred hose having a wall thickness of approximately one millimeter, and made from a thermoplastic polyurethane compound. An example of such hose is disclosed having a larger inside diameter of about 12.1 mm, and tapering down to a smaller inside diameter of 9.6 mm at its opposite end. This tapered hose is to be drawn over the tapered wire of a helical compression spring. Schwarz et al. also disclose that the hose material is to be made of an elastic material that readily recovers its shape, such that the hose ultimately will rest tightly against the wire of the coil spring, thereby allegedly ensuring that any anti-corrosion agent and lubricant inside the hose will not leak out, and that moisture and dirt outside the hose will not penetrate in.
Unfortunately, Schwarz et al. do not disclose the fact that its preferred material, polyurethane, does not have sufficient resiliency to perform the functions required. It has been observed that in actual use, the polyurethane material preferred in Schwarz et al. will not seal tightly around the wire of the coiled spring once it has been run up that spring. In fact, the ends of such a hose will be permanently deformed by the stress of being run up the coiled wire of the spring (i.e, from the successive bending around the coils) and will form air gaps between the inner diameter of the hose and the diameter of the spring wire at both ends of the polyurethane hose.
Because of these air gaps, the polyurethane hose preferred in Schwarz et al. must be sealed by a compound after it has been installed onto the wire of the coil spring. Otherwise, corrosion and dirt will enter the interior portions of this polyurethane hose, and lubricating and anti-corrosive compound will tend to leak out.
In addition, it is significantly more expensive to manufacture tapered hoses such as those disclosed in Schwarz et al. than non-tapered hoses. It is also more expensive to manufacture pre-cut hoses which are cut-to-length at the hose manufacturing facility (before shipment to a coil spring manufacturing facility for final assembly) than to manufacture one long section of hose, which can be shipped from the hose manufacturing facility on a large reel.